Anti condensation heater sizing for a sealed enclosure

Size the heater from the walls, not from the box volume: hold the interior about 10 K [18 °F] above the coldest overnight ambient, and 304 or 316L stainless gives up 3.7 W per square metre of wall for every kelvin of lift. A 20 x 16 x 8 in washdown panel carries about 8.4 ft² [0.78 m²] of outside surface, so it loses 29 W at that lift; the order is a 60 W heater, a thermostat or hygrostat, and a manufacturer-rated vent drain. The steps below run the arithmetic for your box, set the control, and check the result after the first cold night. The same method covers a sealed HMI cabinet or an idle pump panel; only the area, the material and the night low change. If you are still deciding whether the box needs heat at all, start with when an outdoor enclosure needs an anti-condensation heater; the condensation guide holds the dew-point basics.

How many watts keeps a sealed enclosure dry?

Work from Q = k · A · ΔT — wall heat-transfer coefficient times outside surface area times the lift over ambient — which for 3.7 W/m²K stainless on the 20 x 16 x 8 in example box gives 29 W to hold 10 K [18 °F] above a cold night.

Condensation lands on any surface at or below the dew point, and overnight the walls are the coldest surfaces in the box. They radiate to a cold sky or a cold hall wall and settle a few kelvin below the air. Holding the air 10 K above ambient keeps every wall clear of the dew point even then. That is why enclosure heater guidance settles on a +10 K margin between interior and ambient as the working figure. The margin prices out in a straight line: ask the example box for 20 K and the loss doubles to 58 W.

Internal heat counts against the heater. A panel that idles powered gives off its own 10-20 W, and the heater only tops that up. A washdown panel switched dead at the breaker gives nothing, and that is the case sized here. A NEMA 4X / IP66 rating keeps hose water out, but it has no say over the moist air sealed inside — that air is what the heater has to hold above its dew point (why a sealed box still condenses).

These k values are the steady-state wall figures used across this site's enclosure sizing, for thin walls with calm air on both sides. Wind on an outdoor wall raises the loss, and the margin in step 5 exists to cover it.

What do you need before you size the heater?

Before any arithmetic you need four numbers: the outside height, width and depth in inches, the coldest overnight ambient the wall actually sees, the wall material with its k value (3.7 W/m²K for stainless, 5.5 W/m²K for painted steel, 3.5 W/m²K for plastic), and the control you will run.

Gather them like this:

• Dimensions. Measure the closed box outside, in inches; the 20 x 16 x 8 in wall-mount box is the running example.
• The night low. Use the coldest ambient the wall sees: plant records indoors, the local weather record outdoors. A chilled food hall can hold 8 °C [46 °F] overnight; an outdoor wall in a continental winter can sit at -15 °C [5 °F].
• Wall material. Stainless, painted steel or plastic, which fixes the k value.
• The heater itself. A plain convection rail handles duty below about 150 W; above that, thermal-management selection guides expect a small axial fan on the heater so warm air reaches the roof. Mount any rail in the lower third of the back wall, clear of wire bundles.
• The control. A bimetal thermostat, a mechanical hygrostat, or both on the DIN rail, wired into the heater supply.
• A rated vent drain. It adds no watts; it equalizes the pressure swing that washdown heat and night cooling create, and it is what keeps water out while the box breathes.
• A min/max thermo-hygrometer. It does the proving work in the checks section.

For mounting, wiring and thermostat placement inside a full control cabinet, the control cabinet anti-condensation heater setup article covers the install; this page stays on the numbers. And seal before you heat: a tired door gasket trades your warm dry air for saturated process air all night, so replace it before you buy watts.

How do you size an anti condensation heater step by step?

Multiply the six faces into an outside surface area, multiply that by the wall coefficient and the 10 K [18 °F] lift, add about 50% for weather, and buy the next catalog heater size above the result.

1. Measure the closed box. Height, width and depth outside, in inches: 20 x 16 x 8 in here.
2. Add the six faces. 2 × (20 × 16) + 2 × (20 × 8) + 2 × (16 × 8) = 640 + 320 + 256 = 1,216 in². Divide by 144: 8.4 ft² [0.78 m²].
3. Apply the wall coefficient. 0.78 m² × 3.7 W/m²K = 2.9 W per kelvin of lift for stainless.
4. Apply the lift. 2.9 W/K × 10 K = 29 W of continuous heat for a dry box.
5. Trim for internal heat, then add margin. The example panel sits dead overnight, so nothing comes off; powered idle electronics of 10-20 W would. Add about 50% for wind, wet walls and a clear cold sky: 29 W becomes about 44 W.
6. Buy the next catalog size. Enclosure heater catalogs step at 30, 60, 100, 150 and 250 W, so the order is a 60 W rail with its control. Isolate the panel and prove it dead before you wire anything in.

The 60 W step delivers 60 W ÷ 2.9 W/K ≈ 21 K of lift on the example box — the 10 K design margin and the weather margin in one catalog size.

The rows take 3.7 W/m²K stainless and calm air. The 250 W row holds its big box about 14 K above ambient (250 W ÷ 17.7 W/K), which is the design lift plus weather margin in one step, and it crosses the fan crossover at about 150 W. Below about 30 W, the first row's territory, the small enclosure anti-condensation heater selection article walks through the compact rails and their mounting.

How do you set the thermostat or hygrostat?

Set a thermostat to cut in about 10 K [18 °F] above the coldest overnight ambient — nights at 10 °C [50 °F] mean a 20 °C [68 °F] setpoint — or run a hygrostat that switches at about 65% RH, the level above which corrosion and leakage currents climb on electronics.

Temperature control is the insurance policy. It fires whenever the air falls toward the dew point, whether or not the air reads as wet, so it is the single control to trust in a washdown plant. A hygrostat switches on humidity alone and saves watts where the air runs dry, but it samples the air while your walls are the surfaces that actually cool. Many washdown panels run both devices in parallel, so either contact calls for heat.

After washdown the job changes from holding a margin to drying a film. Water needs 2.26 kJ to evaporate per gram (the steam-table figure at atmospheric pressure), so 100 g of washdown water on walls and components absorbs 226 kJ. A 60 W heater supplies that in about 63 minutes when every joule reaches the film. Count on two to three hours once wall losses take their share. Motors idling on the same line dry out the same way, with a small heater warming the windings; preventing motor condensation with anti-condensation heaters covers that variant of the problem.

What checks prove the heater works?

After the first cold night, read three numbers: the inside minimum temperature about 10 K [18 °F] above the outside minimum, interior RH below 65% at the morning low, and no droplets under the roof when you open the door.

• Temperature pair. A min/max thermometer inside and a second one at the wall: the inside minimum should sit about 10 K above the outside minimum through the coldest hour.
• Humidity log. Leave a thermo-hygrometer inside for a week. Interior RH that stays below 65% at the daily low means the margin holds; a morning spike past it means a size step is missing or moisture is walking in at the gasket.
• The morning look. Roof, door top rail and terminals dry at opening time. A damp gasket groove points at the entry path, not at the wattage.
• Duty cycle. Clamp the heater feed at the design night low. A heater that never clicks off at the design ambient is one catalog step short or fighting a leak; fix the leak first, then step up.
• After the first washdown. Repeat the morning look the day after a full hose-down followed by a cold night; that pair of events is the real test.

A cabinet that stays closed and idle between service visits can carry the night on desiccant instead of watts; the desiccant sizing article works out how many grams a sealed box needs.

Which mistakes cost you the rating or the heater?

The costly three are an open drain hole that strips the Type 4X rating, an uncontrolled heater that bakes gaskets and insulation, and a wattage picked from box volume instead of wall area.

• A bare drain hole. NEC 314.15 permits approved drainage openings of 3 mm [1/8 in] to 6 mm [1/4 in] in boxes listed for damp or wet locations. Approved there means a drainage fitting, not a drill bit. On a hose-down box the fitting to use is a manufacturer-rated vent drain. Liquid drains out, the hose spray stays out, and the pressure swing of night cooling equalizes without dragging water past the gasket.
• The weakest opening. Whatever you fit, the part with the lowest rating sets the assembly. Our stainless boxes are manufacturer-rated NEMA 4X / IP66; they are not UL listed, so each fitted device has to carry the same manufacturer rating for the assembly to hold Type 4X.
• An uncontrolled heater. A bare rail wired live runs 60 W against the door seal around the clock. Silicone gaskets tolerate about 200 °C, but wire insulation and terminal plastics age fast, so put the thermostat or hygrostat in series with the heater supply.
• Volume thinking. Watts follow area, not volume. Going from a 20 x 16 x 8 in to a 20 x 16 x 12 in box adds 50% to the volume: 2,560 to 3,840 in³. It adds only 24% to the wall, 1,216 to 1,504 in², and the wall is what the heater sees.
• Blocked convection. Mount the rail in the lower third with clear air above it. A loom strapped across the heater bakes the cable insulation and starves the roof of warm air.
• Moisture left standing. Warm, wet, warm again is the cycle that pits even 316L in a gasket crevice; why wet-dry cycling rusts 316L before you blame the grade shows that failure and its fixes.

The decision, in one line: wall area times 3.7 W/m²K per kelvin times 10 K of lift, plus half again for weather, rounded up to the next catalog size. A 60 W heater covers most wall-mount washdown panels that sit dead overnight. For a box drawn with the heater, gland and vent-drain cutouts where you want them, custom stainless enclosures go from your approved drawing to delivered in 10 business days, and the stainless size finder filters the catalog range from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D) by grade and size.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions